Axial trajectory sensor for electronic particle study apparatus and method

ABSTRACT

A sensor for use with apparatus operating in accordance with the principles of the Coulter electronic particle studying device, for differentiating between signals from particles passing on axial or near axial paths through an aperture and particles passing off center. The pulse duration is measured at some fraction of the individual pulse amplitudes and only those which meet the criteria of duration established by the electronic circuitry are permitted to pass for use in pulse height analysis apparatus following the sensor. The other pulses are discarded on the basis of their greater durations. The apparatus of the invention provides structure for deriving a duration-measuring pulse whose duration is that of the particle pulse at its fractional amplitude, structure for converting the signal into one which has an amplitude proportional to duration, and then measuring the latter signal against a certain maximum level to operate gating means automatically for rejecting the longer duration pulses and passing the shorter duration pulses. The maximum level is automatically produced by structure which responds to the level of prior passed pulses thereby establishing a maximum duration which when exceeded will cause a pulse to be discarded.

United States Patent [151 3,700,867

Gogg Oct. 24, 1972 AXIAL TRAJECTORY SENSOR FOR Erargtiner- -Malcolm A.Morrison ELECTRONIC PARTICLE STUDY APPARATUS AND METHOD [72] Inventor:Walter R. Gogg, Miami Lakes, Fla.

[73] Assign'ee: Coulter Electronics, 102., Hialeah,

Fla.

[22] Filed: Dec. 24, 1970 [21] Appl. No.: 101,325

Related US. Application Data [63] Continuation-impart of Ser. No.84,440, Oct.

[52] US. Cl ..235ll51.3, 235/92 PC, 307/234, 324/76 R, 328/112 [5 1]Int. Cl. ..ll03k 5/18 [58] Field of Search...235/92 PB, 92 PC, 92 MT,

151.3, 235/l5l.3l', 307/234; 328/111, 112; 324/76R References CitedUNITED STATES PATENTS 3,44] ,848 4/1969 Valley et a]. ..235/92 PB XAssistant Examiner-R. Stephen Dildine, Jr. Attorney-Silverman & CassABSTRACT A sensor for use with apparatus operating in accordance withthe principles of the Coulter electronic particle studying device. fordifferentiating between signals from particles passing on axial or nearaxial paths through an aperture and particles passing off center. Thepulse duration is measured at some fraction of the individual pulseamplitudes and only those which meet the criteria of durationestablished by the electronic circuitry are permitted to pass for use inpulse height analysis apparatus following the sensor. The other pulsesare discarded on the basis of their greater durations. The apparatus ofthe invention provides structure for deriving a duration-measuring pulsewhose duration is that of the particle pulse at its fractionalamplitude, structure for converting the signal into one which has anamplitude proportional to duration, and then measuring the latter signalagainst a certain maximum level to operate gating means automaticallyfor rejecting the longer duration pulses and passing the shorterduration pulses. The maximum level is automatically produced bystructure which responds to the level of prior passed pulses therebyestablishing a maximum duration which when exceeded will cause a pulseto be discarded.

Mitt/0? aA/E Mar PMENTEDMI 24 m2 SHEH 1 OF 8 ATTORNEYS PATENTEDncr 24 m2SHEET 8 BF 8 AXIAL TRAJECTORY SENSOR FOR ELFXITRONIC PARTICLE STUDYAPPARATUS AND METHOD CROSS REFERENCE TO RELATED APPLICATION Thisapplication is a continuation-in-part of my copending application havingthe same title, Ser. No. 84,440 filed Oct. 27, 1970.

BACKGROUND OF THE INVENTION The field of this invention is particleanalyzing apparatus and more particularly is concerned with apparatus inwhich studies may be made of particulate systems using the Coultersensing principle in a manner to obtain more accurate size informationthan heretofore achieved.

The Coulter sensing principle is disclosed in US. Pat. No. 2,656,508issued Oct. 20, 1953 to Wallace H. Coulter. According to this principle,when a microscopic particle in suspension in an electrolyte is passedthrough an electrical field of small dimensions approaching those of theparticle, there will be a momentary change in the electric impedance ofthe electrolyte in the ambit of the field. This change of impedancediverts some of the excitation energy into the associated circuitry,giving rise to an electrical signal. Such signal has been accepted as areasonably accurate indication of the particle volume for mostbiological and industrial purposes. Apparatus embodying the teachings ofUS. Pat. No. 2,656,508 has been used to count and size particles inbiological fluids, industrial powders and slurries, etc.

The principles of the present invention apply to Coulter particleanalyzing apparatus in which the excitation of the field is achieved bymeans of unidirectional or low frequency power sources or radiofrequency power sources.

In commercial versions of the Coulter particle analyzing apparatus, theelectric field of small dimensions has been formed commonly by amicroscopic right cylindrical passageway or aperture, as it is known,between two bodies of liquid in which the particles to be studied aresuspended. The electrical excitation energy is coupled to these bodiesby means of electrodes respectively located in the liquid bodies, theaperture being formed in an insulating wall between the bodies. Thesuspension is caused to flow through the aperture carrying the particleswith the flow and giving rise to the electric signals produced by themomentary changes in impedance caused by the respective particles asthey pass through the aperture. The electric field is concentrated inthe aperture and normally comprises an electric current flowing throughthe aperture along with the physical fiow of suspension.

By counting the signals produced, one can count the particles passingthrough the aperture. By discriminating between different pulseamplitudes, one can make size studies. This invention is primarilyconcerned with size studies, and has, as a very important objectthereof, the provision of apparatus which will enable highly accurateparticle size data to be achieved.

lt has been known that long apertures can produce results which aresuperior to short apertures insofar as size measurements are concerned,if the bandwidths of the associated amplifiers are reduced accordingly.A long aperture may be considered one in which the length is greaterthan the diameter. The usual Coulter aperture is relatively short, thatis, its length is the same as or less than its diameter.

The reason for better size information with long apertures is that theelectrical field halfway through the aperture, being the position mostremote from the entrance and exit of the aperture, is most uniform andhas the most uniform current distribution for all paths through theaperture. The longer the aperture, the more nearly uniform is the fieldat this midpoint. At the entrance and exit of the aperture, the currentdensity is greater at the edges of the aperture and correspondinglylesser on the axis of the aperture. This may be explained by pointingout that current paths other than the axial path are supplied from thesides of the aperture as well as straight ahead. The lower currentdensity on the axis at the entrance and exit results in a lowerinstantaneous signal than is the case for particles entering theaperture and leaving it on other paths. In other words, the currentdensity at the corners of the aperture is greater than at the axis.

Another phenomenon is important to consider, according to thisinvention. The velocity of electrolyte flow, and hence the velocity ofparticles also, is somewhat greater on an axial path than on pathscloser to the edges of the aperture or paths which are offcenter,because the liquid does not have to change direction when it goesthrough the axial center of the aperture. The resistance to flow is aminimum on the axis since it is surrounded by a moving sheath of liq uidhaving substantially the same velocity.

The prior art has recognized the problem involved in the use of theCoulter apparatus for sizing studies, but so far as is known, there hasbeen no satisfactory solution. One attempt involved releasing theparticles in a suspension from a focussed source ahead of the aperture;but this involved the use of two apertures and the inability toilluminate and view the aperture during the process.

The use of long apertures poses too many problems to make the samepractical. The long aperture has less sensitivity. It adds resistance tothe effective aperture which generates noise tending to mask thesignals. Microphonic modulation of the aperture is also increased. Thelong aperture is more likely to have coincident particles in it, givingrise to counting and sizing errors. The long aperture is more likely tobecome blocked by debris and is not as easy to clear as a shortaperture. The flow rate of long apertures is decreased because ofincreased resistance to flow, thus decreasing the time for making anygiven studies.

The problems referred to above are solved by the basic invention whichis disclosed in the previously mentioned co-pending application. Certainother problems with respect to the invention are additionally solved bythe invention herein. The disclosure of the co-pending applicationutilizes manually controlled circuitry to establish the principalthreshold for determining whether a given signal will be sensed or not.This requires testing and adjustment procedures which must be undertakenfor practically every use of the apparatus. The invention hereinutilizes circuitry which obviates the need for manual adjustment, butinstead has an automatic memory device which responds to the duration ofcertain previously passed signals, and hence keeps itself adjusted to acertain duration level.

Other refinements of the invention herein are concerned with preventingthe sensor from operating during the processing of a pulse, so that partpulses will not trigger the sensor and so that one pulse followingimmediately after another will not operate the sensor until the analysisis complete.

The invention herein also teaches the transmission of complete pulsesthrough the sensor, as opposed to the transmission of synthetic pulsesderived from complete pulses.

SUMMARY OF THE INVENTION According to the invention, particles passingthrough an aperture are examined electronically to ascertain which ofthem passed most nearly on axial paths through the aperture. These arethe only particles which are permitted to be regarded by the apparatus,the others being disregarded. The electronic selection is based on thefact that the particles following axial paths spend the least time inthe ambit of the aperture, and therefore their corresponding pulses havethe shortest duration. Theoretically, all pulses passing through theaperture, regardless of size, will have the same duration; but becauseof the reasons given above, this is not practically true. Pulses whichpass through the aperture off-center will normally have longerduratrons.

By disregarding a percentage of the pulses, fewer are considered by thepulse height analyzing equipment which follows the sensor of theinvention, resulting in a slight degradation in the statistical accuracyif a given amount of sample of a given concentration is scanned. Thedata which are achieved, however, are of much higher quality. If a countis required, this is made before the signals are processed in the sensorof the invention.

The particle pulses are examined by ascertaining their durations at somefraction of their amplitudes. The resulting measuring signal is thenconverted into a pulse whose amplitude is proportional to the durationof the measuring signal. This amplitude is then compared with certaincriteria to ascertain whether the original pulse was of a size to bepassed to the pulse height analyzing equipment or to be disregarded.

In this invention, the criteria are established by means of a minimumduration memory circuit which responds to the minimum pulses which areapplied to the sensor. The circuit remembers the duration of theshortest of the previous pulses and does not pass any pulse whoseduration is some predetermined small amount greater. Structure isprovided to disable the sensor during the processing of a pulse toprevent application of any following pulses or part pulses whileprocessing is occurring. Structure is also provided to disable thesensor unless the incoming signal has dropped below a predeterminedthreshold, to prevent processing of partial pulses.

A novel circuit for automatic adjustment of the maximum duration levelis provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic view of theaperture of a Coulter particle analyzing apparatus showing the paths ofdifferent particles through the apparatus;

FIG. 2 is a diagram showing the graphs of particle pulses resulting fromthe passage of the particles of FIG. 1 along the paths shown through theaperture;

FIG. 3A is a block diagram of an axial trajectory sensor constructed inaccordance with the invention;

FIG. 3B is a diagram consisting of a series of graphs all on the sametime scale illustrating various wave shapes throughout the sensor ofFIG. 3A resulting from the processing of two particle pulses therein;

FIG. 4A is a block diagram of an axial trajectory sensor of a modifiedform;

FIG. 4B is a diagram consisting of a series of graphs all on the sametime scale illustrating various wave shapes throughout the sensor ofFIG. 4A resulting from the processing of two particle pulses therein;

FIG. 4C is a detailed block diagram of the logic control circuit ofFIGS. 3A and 4A;

FIG. 5 is a fragmentary block diagram of an axial trajectory sensor ofstill another modified form; and

FIG. 6 is a block diagram of a system constructed in accordance with theinvention and using an axial trajectory sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is based upon apractical consideration of the electric pulses which result when asuspension of particles is passed through the aperture of a Coulterelectronic particle analyzing device. Since the physical length of theaperture is constant, and one would consider that the rate of flowremains constant, it should follow that all particle pulses, that is,the electrical signals produced in the detector of the Coulterapparatus, should have the same duration. That this is not true has beenknown for some time. A particle passing through the aperture on anangle, such as for example, entering close to an edge of the entranceand/or passing close to a wall of the aperture, will not have the sameduration as the identical particle entering on the axial center andpassing through the center of the aperture. Additionally, the amplitudeand/or profile of the signal may be considerably different from thesignal produced by the on-center particle.

As explained in the co-pending application, the basic concept of theinvention therein and that which is disclosed in this application is tomeasure the pulse duration of signals produced by the particles passingthrough the aperture and then to discriminate between these signals insome way, discarding the longer of them and using only the shorter ones.The circuitry of the sensors of the invention herein and that of thecopending application embody the methods for doing this and utilizecertain novel structure respectively for applying criteria fordiscarding some and using others. Although detailed in the co-pendingapplication, it would also be of value here to explain the differencesbetween the types of pulses which are produced by particles passingthrough the aperture of a Coulter electronic particle analyzing device.

A system constructed in accordance with the invention is illustrated inFIG. 6. The block 10 comprises a Coulter particle analyzing apparatuswhich is normally composed of a stand, detector and counter. The standincludes the vessels, aperture tube, fluid system and electrodes of theapparatus. The detector includes circuitry which produces the particlepulses. The counter may be any device which responds to the particlepulses, and may include pulse height discriminating means. It may beomitted in instances where only size studies are to be made, but isshown in order to point out that since the sensor 3-10 will bediscarding many pulses, it is best to make any counts prior to applyingthe particle pulses to the sensor 3- 10. As seen, from the sensor 3-10,the output signals at 3-78 are applied to some form of pulse heightanalyzer 14 in order to make the sizing studies.

FIG. 1 is a diagrammatic view of an aperture which constitutes thescanning means in the stand of Coulter electronic particle device 10,immersed in a liquid and having particles passing through the apertureof the wafer. Thus, the wafer is designated 20, and the aperture itselfis designated 22. The sample liquid is passing through the aperture 22from right to left, and as it moves, it carries the particles insuspension with it. The paths of three particles, X, Y, and Z, areillustrated at 24, 26, and 28, respectively. These paths aredeliberately chosen to be considerably different, for purposes ofillustration, and the signal or particle pulses which are produced as aresult of such passage are shown on the identical time base in FIG. 2 atgraphs X, Y, and Z.

The particle X passes almost coaxially of the aperture 22 along the path24. The speed of the liquid passing through the aperture at this pointis maximum and the current density distribution along the path is mostuniform. Accordingly, the resulting pulse 30 in FIG. 2, as shown in thecurve X, is a simple bell-shaped pulse whose duration is proportional tothe length of the aperture 22 from t, to t, and whose amplitude is quiteclosely proportional to the size of the particle. Although the amplitudewill be considered as voltage, it should be understood that pulses andsignals could also be current waves.

The particle Y passes through the aperture 22 on a diagonal path 26. Inthe first place, it will be appreciated that its path, while traversingthe aperture, is longer than the path 24 because it is at an angle. Inthe second place, at the point where it entered the aperture, this beinga corner at 32, the current density is much higher than that closer tothe axis of the aperture. Accordingly, the beginning of the pulse 34which is produced by this particle, will have a higher amplitude, andwill also probably commence slightly before the pulse 30. If itcommences at practically the same time 1,, due to its time within theaperture being longer, it will finish later than the time As shown,there is a peak at 36 due to the effect of high current density at thecorner 32, and a lesser peak at 38 which is produced when the particleleaves the aperture, since it is approaching the high current density atthe comer 40.

The particle Z goes through the aperture 22 on a relatively straightline, but in this case it is quite close to the wall 42 of the aperture.The resulting pulse 42' has two peaks, one at 44 caused by the corner 32with its high current density, and the other at 46 caused by theparticle passing the high current density corner 48. In this case, theparticle will remain in the aperture longer than the time t, to becausethe speed of flowing liquid is less adjacent the wall than it is in thecenter of the stream. This is a well-known phenomenon of flow of liquidsthrough orifices.

In these three cases, it can be seen that the only pulse which is mosttruly representative of the size of the particle is that which passesthrough the center of the aperture 22, namely, the particle X. Accordingto the invention, circuitry is provided to discard pulses of the othertypes, based upon their time duration, since it becomes clear that onlythe pulses of shorter duration have gone through the center of theaperture and produced the most representative wave shapes.

According to the invention, structure is provided to discriminatebetween the different types of pulses which are illustrated in thegraphs of FIG. 2. The basis for discrimination in the apparatus which isdescribed in connection with the several circuits detailed hereinafteris analog in nature, although as explained in the co-pendingapplication, the basic concept which underlies both the invention hereinand that of the said co-pending application can be applied to digitallyoperating apparatus as well.

The block diagram of FIG. 3A illustrates an axial trajectory sensorwhich utilizes the basic concept and invention of the said co-pendingapplication and in addition is constructed in accordance with thisinvention to achieve the purposes alluded to above. For the purposes ofthe explanation, it is assumed that two particles are being examined,such as particles X and Y of FIGS. 1 and 2 and the resulting wave shapesderived from the pulses throughout the circuit are illustrated in FIG.38, all on the same time base.

The apparatus of FIG. 3A is designated generally 3-10 and ischaracterized by the provision of means to discriminate between pulsesof difierent durations, on the basis of which the desired pulses arechosen and permitted to pass through the apparatus, albeit in analogform. This discriminating means is automatically controlled by thepulses themselves, and operates on the assumption that practically allof the desirable pulses will have the same duration and that thisduration will be the maximum duration tolerated. All pulses whosedurations exceed this maximum will therefore be discarded, and allpulses which are less than this maximum duration will be accepted. Atolerance of duration is built in the circuitry so that pulses whosedurations are slightly greater than the maximum will nonetheless beaccepted.

This same discriminating means is used in the structures 4-10 and 5-10of FIGS. 4A and 5 respectively, being common to all the structures whichare described herein. In addition, there are means for disabling theapparatus during the processing of a pulse, the details of which will beexplained. The purpose of the latter means is to prevent partial pulsesof very small duration from causing an undesirable long-term reaction inthe discriminating means, as for example, would occur if the apparatusis receptive too soon after a pulse has been processed, and a particlepulse immediately following is accepted after a part has already beenheld back. Simple threshold means can be used to prevent noise fromaffecting the discriminating means.

The output pulse of the sensor 3-10, in the event that one appears, inevery case will have the identical amplitude as the original particlepulse, such as for example, the pulse 30 of FIG. 2, but will have apredetermined duration governed by the electronic characteristics of thecircuitry. Since size information is paramount, the duration of theoutput pulse is immaterial, and according to the operation of theapparatus 3-10, all output pulses will have the same duration. This isalso true of the apparatus -10 of FIG. 5.

In the case of the apparatus 4-10 of FIG. 4A, instead of an analog ofthe pulse input, the output pulses are actually reproductions of theinput pulses, achieved by delay means as will be described.

Referring now to FIGS. 3A and 3B, the input terminal 3-12 has applied toit the train of pulses which emanate from the apparatus 10 that isconstructed in accordance with the principles of US. Pat. No. 2,656,508.As such, these pulses have been produced as a result of particlespassing through an aperture such as 22 illustrated in FIG. 1, and are ofdifferent amplitudes, durations and configurations. Ideally, the pulsesemerging from a Coulter electronic particle device should all bebell-shaped (assuming that the conventional short aperture is used),should all have the same duration and should differ from one anotheronly as to their amplitude. Practically, this does not always occur. Theinvention herein and that of the co-pending application are based uponthe practical limitations of the usual Coulter device and seeks torender the information on size obtainable therefrom more accurate andreliable.

The train of pulses which is received at the terminal 3-l2 has beenamplified prior to application to the apparatus 3-10, this beingaccomplished by circuitry normally included in the detector of the usualCoulter electronic particle counting and sizing apparatus 10 or evenafterwards.

The signals appearing at the input terminal 3-12 are the particle pulsessuch as illustrated in FIG. 2. The two pulses 3-30 and 3-34 shown ingraph A of FIG. 38 represent the desirable and the undesirable pulses,respectively. It is desired to accept the first and reject the second,the output at 3-78 being an analog of the first pulse and there being nopulse at all corresponding to the second pulse. The first pulse 3-30comprises a pulse produced by a particle which traversed the aperture 22as approximately the center thereof, while the pulse 3-34 being causedby a particle which went through the aperture off center.

Considering first the pulse 3-30, it has an amplitude X and, assumingthat it has passed through the analog gate 3-20, it appears on the line-22 and is applied simultaneously to the pulse stretcher 3-26 and theanalog signal delay means 3-24. The capacitor of the pulse stretcher3-26 follows the leading edge of the pulse 3-30, charging up to theamplitude X and holding this charge after the pulse 3-30 subsides. Theresult is the flat-topped pulse 3-38 of graph B of FIG. 3B, having anamplitude X and a duration controlled by other elements of the circuit.This pulse appears at the line 3- 36 to be applied to the attenuator3-40 and to the line 3-156 which extends to the output analog gate 3-76,this latter being closed at the time the pulse 3-38 cornmences. Theattenuator 3-40 attenuates the pulse 3-38 to some desired fractionalvalue of the amplitude X and the resulting fractional amplitude pulse342 of the graph C appears at the input line 344 of the comparator 3-46.In the meantime, the pulse 3-30 has been delayed by the analog signaldelay means 3-24 and appears as the pulse 3-32 on the line 15-50, beingthe second input to the comparator 3-46. The fractional amplitude forpulse 3-42 is chosen so that a sharp comparison can be made with pulse3-32. Conveniently, this can be 50 percent, but other fractions can beused.

The two pulses 3-32 and 3-42 are shown superimposed on the graph C ofFIG. 38. Where the pulse 3-32 exceeds its own fractional height asmeasured by the attenuated pulse 3-42, namely between the times t, and tthe comparator 3-46 will have an output that appears as the rectangularpulse 3-54 at the line 3-52, shown in graph D of FIG. 3B. This is aninput to the veto AND gate 3-60. It is also an input on the line 3- 103to the trailing edge detector 3-102. The output of this detectorcomprises the spike 3-104 of graph F which is delayed by the duration ofthe pulse 3-54 to the time It is applied to the lower input of the vetoAND gate 3-106, and if the gate is open, passes to the first one-shotmultivibrator (A) 3-108 of the timing sequence generator to produce arectangular output pulse 3-96 (graph G) between the times t, and t;,. Inthis same manner, the timing pulses 3-98 and 3-100 are produced at thetimes shown in graphs I and K of FIG. 3B. Spikes 3-114 and 3-134 ofgraphs H and .l are produced by the respective trailing edge detectors3- 112 and 3-131. The spike 3-114 appears at the line 3- 116 andtriggers the one-shot (B) 3-118, while the spike 3-134 appears at theline 3-136 and triggers the one-shot (C) 3-138. The timing pulse 3-98appears at the lines 3-120, 3-132, 3-127, 3-122 (which extends back tothe veto input of the veto AND gate 3-106), 3- 64, 3-128 and 3-126.

The timing pulse 3-98 appears at the line 15-64 to the veto input of theveto AND gate 3-60 and this closes the gate, but note that this occursat the time to Accordingly, since the duration measuring signal 3-54occurs at time t, to l it passes through the gate 3-60 and appears atthe line 3-62, being applied to the integrator 3-58. The integrator 3-58integrates the comparator output 3-54 and produces a ramp and pedestalpulse 3-66, the pedestal or plateau extending from the time when thecomparator signal subsides to the time The timing sequence generator3-56 is energized at the end of the comparator output pulse 3-54 andholds off all further incoming signals, until the measurement cycle iscompleted. This will be explained below.

It will be noted that the output of the one-shot (C) 3- 138 whichcomprises the timing pulse 3-100 commencing at the time I, (see graph K)and extending for any desirable time to 1 is applied by way of the lines3-140 and 3-142 to the pulse stretcher 3-26, the shorting means of theintegrator 3-58 and to the control logic circuit 3-16. The pulsestretcher and integrator are thus reset by the signal 3-100 between thetime t, and

At the end of the delay one-shot pulse 3-96, as previously explained,that is, at the time 1 the trigger spike 3-114 is produced from thetrailing edge detector 3- 112 at the line 3-116 and triggers theone-shot (B) 3- 118. This is the strobing one-shot multivibrator of thesensor 3-10. It inhibits further trigger spikes from coming through theveto AND gates 3-l06and 3-60 and it also opens the AND gates 3-88 and3-130. The efi'ect of this will shortly be described. It can be readilyseen that if this strobing pulse 3-98 can pass through the AND gate3-130, it will become the pulse 3-200 (graph N) on the line 3-158, andwill open the analog gate 3-76 between the time t;, and t and therebypermit a portion of the pulse 3-38 to pass to the terminal 3-78. Theresulting output pulse 3-202 (graph has the amplitude X and the durationt, to

Whether or not the strobing pulse is effective depends upon the minimumduration memory circuit 3- 70 which is the discriminating means todetermine whether the strobing pulse will pass.

The signal pulse 3-66 appearing at the line 25-68 is applied to thecomparator 3-150 and to the broken line block of FIG. 3A which isdesignated 3-70, this block comprising elements forming a minimumduration memory circuit. The output of this circuit 3-70 appears at 3-72and such output comprises a dc. voltage proportional to the minimumduration. This is the lower of the two broken line wave forms of graph Edesignated 3-74. The details of operation of the minimum duration memorycircuit will be set forth below, but for the time being, it should betaken that it controls whether or not the pulse output from theintegrator 3-58 (wave form 3-66) is effective to permit a portion of thestretched pulse 3-38 to pass through the last analog gate 3-76 to theoutput terminal 3-78. lf the amplitude of the pulse 13- exceeds thevoltage level of the line 3-72 by an amount controlled by the gain ofthe adjustable gain amplifier 3-80, the upper level being shown by theupper one of the broken line wave forms of graph E of FIG. 3B, namely3-82, then the analog gate 3-76 will be closed, that is, the precisionelectronic switch which comprises this gate will be opened. Under thesecircumstances, no signals can get through to the output terminal 3-78.The situation is represented by the pulse 3-220 and the resultingeffects.

The minimum duration memory circuit 3-70 is composed of an inputcomparator 3-84 operating into one terminal 3-86 of an AND gate 3-88whose output 3-90 is applied to the shorting means of the integrator3-92. The constant current supply 3-94 may consist of a constant voltagesource and fixed resistor, the output of which is applied to theintegrator 3-92 to cause the voltage at the output 3-72 to rise veryslowly. In the graph E, the slope is exaggerated in the wave form 3- 74.It is only necessary that the constant current supplied exceed anyleakage at the input to the integrator 3-92 to ensure that theintegrator always has a tendency to drift upward and never downward.

The pulse 3-98 from the one-shot 3-118 provides an input to the ANDcircuit 3-88 by way of the line 3-132 after the integrator pulse 34% hasstopped increasing. If, at this time, the voltage at 3-68 is less thanthe voltage at 3-144, the comparator 3-84 has an output at 3- 86 whichcombines with the signal at 3432 to produce an output at 3-90 which isshown as the pulse 3-146 of graph L. When this happens, the dischargingmeans of the integrator 3-92 is energized, causing the voltage at theoutput 3-72 to decrease between time t; and r, in graph E. When thevoltage falls below the voltage at 3- 68 at time the output from thecomparator 3-84 disappears, removing the energizing signal 3-146 fromthe AND gate 3-88 and the shorting means of the integrator 3-92, thuscausing the integrator output at 3-72 to hold at the new value which isthat of the pulse at 3-68.

If the pulse at 3-68 is of greater amplitude than the voltage at 3-144,as is the case with the pulse 3-220, the comparator 3-84 will not havean output at 3-86, the shorting means of the integrator 3-92 is notactivated, and the voltage at 3-72 is not affected. in this manner,since only pulses having durations shorter than the previous minimumcause the integrator output at 3-72 to decrease, the voltage at 3-72 isproportional to and represents the duration of the previous shortestpulse. This establishes a maximum duration.

The voltage derived and appearing at the path 3-72 is applied to theadjustable gain amplifier 3-80 which has a gain slightly greater thanunity. This permits pulses slightly longer than the established maximumrepresented by level 3-74 to be accepted by the system and routed to theoutput terminal 3-78. This is accomplished by comparing the time-relatedpulses which appear on the path 3-68 with the dc. voltage appearing onthe path 3-148. If a pulse is slightly longer than the duration of theprior minimum duration pulse by a certain small percent, it will exceedthe voltage at 3-148 if the gain of the amplifier 3-80 is adjusted toexceed unity by this small percentage. Only pulses of shorter durationwill therefore result in an output on the line 3- 164 to be applied tothe AND gate 3-130. The maximum duration level is therefore establishedby the voltage 3-82. Any pulse of longer duration will not get throughthe sensor 3- 10.

The adjustment of the amplifier 3-80 is accomplished in this case bysome manual gain adjusting means 3- 162. The voltage level on the line3-148 corresponds to the broken line wave shape 3-82 of graph E of FIG.3B.

The comparator 3-150 has, as one of its inputs, the voltage level atpath 3-148 which is 3-82, and since the voltage plateau 3-66 appearingon the line 3-68 does not exceed that on the path 3-148, nothing happenson the output path 3-164. The output of the comparator 3- is shown ingraph M of FIG. 3B and as seen it is a positive level, meaning that thecomparator 3-150 normally has an output which is discontinued when thevoltage on the upper input 3-68 exceeds that of the lower input 3-148.The addition of the strobe pulse 3- 98 from the path 3428 causes thestrobe pulse 3200 on the line 3-158 thereby connecting the outputterminal 3-78 with the pulse stretcher line 3456 for the duration of thestrobe pulse. This results in the output pulse 3-202.

Now, when a pulse which is not desirable occurs, such as the pulse 3-34,much the same train of events occurs. The ramp of the pulse 3-220 doesexceed the voltage level represented at 3-82 and hence the comparator3-150 switches and removes its output from the path 3-164. This is shownas the negative pulse 3-204 of the graph M commencing at the time t,.Having only one input signal, the AND gate 3-130 ignores the strobepulse 3-98 arriving on the line 3428 and hence the control line 3-158has no pulse equivalent to the strobe pulse 3-200 and the amplitude Y ofthe long duration particle pulse 3-34 is ignored. Obviously, there is nooutput at the terminal 3-78.

Likewise, the voltage at the path 3-68 which is applied to the input ofthe comparator 3-84 exceeds that at the path 3444, as shown in the graphE at the time t and hence the AND gate 3-88 has no input at 3-86 whenthe strobe pulse 3-228 is applied at the time r The shorting means ofthe integrator 3-92 is not energized and the voltages on the paths 3-72and 3-148 remain unchanged. The circuit 3-70 thus continues to rememberthe voltage corresponding to the shortest pulse. No pulse equivalent topulse 3- 146 occurs.

It will be seen that there is an analog gate 3-20 interposed between theterminal 3-12 and the pulse stretcher 3-26. Unless this gate is opened,no signals of any kind can be accepted by the sensor 3-10. Operation ofthis gate is controlled by the control logic circuit 3- 16. This in turnfunctions to keep the sensor disabled so long as there are any signalson the lines 3-110, 3-126 and 3-142. Adjustment of the delays producedby the timing sequence generator 3-56 will enable the adjustment of thetime that the sensor is turned off, with relation to the processing of apulse. As explained, this prevents the arrival of any signals before theprocessing is complete. Circuit 3-16 also prevents part signals fromtriggering the apparatus. The details will be described hereinafter.

The sensor 4-10 which is described in connection with FIGS. 4A and 4Bdiffers from the sensor 3-10 in several respects. The most importantdifierence lies in the type of output which is produced by therespective apparatus. In the case of the sensor 3-10, the output at 3-78is a rectangular pulse, having the amplitude of the particle pulseproducing the same, and the duration of the strobe pulse produced by theone-shot 3-118. In the case of the sensor 4-10, the output is a delayedversion of the original pulse, this being achieved through the use ofdouble delay. The duration of the one-shot strobing pulse from theone-shot 4-118 is long enough to pass all of the delayed particle pulse.In addition, there is a slight difference in the timing means of thecircuit.

The apparatus of FIG. 4A is designated generally by the referencecharacter 4-10. The signal input at the terminal 4-12 comprises thepulses produced by a Coulter type particle apparatus and its associatedamplifiers and power supplies. It comprises a series of pulses whoseamplitudes are ideally proportional to the size of the respectiveparticles causing the same. In all cases, the duration of the pulse istheoretically dependent upon the time spent by the particle in passingthrough the aperture 22. As seen, these durations vary because of theposition of the particle in making the excursion, and the purpose of thesensor apparatus is to choose those pulses which are the shorter induration since these represent the particles which have passed throughthe center of the aperture. From the terminal 4-12, there is a line 4-14extending to the control logic circuit 4-16 mentioned above. The outputof this circuit appears at the line 4-18 and is applied to the analoggate 4-20 which is interposed between the input terminal 4-12 and theline 4-22 leading to the analog signal delay means 4-24 and the pulsestretcher 4-26. The two types of pulses which are considered in thediscussion of the apparatus 4-10 are those illustrated in graph A ofFIG. 4B and are substantially the same as the pulses 3-30 and 3-34 ofFIG. 3B. These are designated 4-30 and 4-34.

For the purpose of logical discussion, it will be assumed at this timethat the analog gate 4-20 lets signals through so that the first pulseto be discussed, namely, the pulse 4-30 is applied to the analog signaldelay means 4-24 and to the pulse stretcher 4-26. The duration of thepulse 4-30 is from the time t, to the time t, and it is delayed by thedelay means 4-24 as seen in graph C of FIG. 48, this delayed pulse beingdesignated 4-32. The output of the pulse stretcher 4-26 appears at 4-36,and as seen it consists of a leading edge that is identical to theleading edge of the pulse 4-30 and a plateau which is equal in amplitudeto the maximum amplitude reached by the pulse 4-30 extended to the time2 This stretched pulse is shown as 4-38 in graph B of FIG. 43. From thepulse stretcher 4-26, the pulse 4- 38 is applied to the attenuator 440which decreases the amplitude of the stretched pulse, resulting in theattenuated pulse 4-42 shown in graph C superimposed on the pulse 4-32.This attenuated pulse 4-42 is applied to the input 444 of the comparator4-46, the other input 4-48 being derived from the line 4-50 and carryingthe output signal 4-32 of the analog signal delay means 4- 24.

From the comparison of the two signals 4-32 and 4- 42, an output appearsat 4-52 which is a rectangular wave 4-54 having a predeterminedamplitude controlled by the characteristics of the comparator 4-46 and aduration from to t, that is controlled by the points at which the pulse4-32 exceeds the pulse 442. As previously explained, the attenuation ofthe stretched pulse 4-38 to produce the pulse 4-42 is a matter of choiceto provide the best results. As shown here, it is chosen as 50 percent.The pulse 4-54 is shown in graph D of FIG. 4B.

The peak amplitude of the pulse 4-30 is stored for a period of time inthe form of a charge on a capacitor, as in the case of the pulsestretcher 3-26 described herein and in said co-pending application. Theamount of delay which is provided by the analog signal delay means 4-24(conveniently an L-C delay line) is chosen to ensure that the pulsestretcher 4-26 has stored the amplitude of the pulse 4-30 before thecomparison is made.

The line 4-52 has two branches, one of which leads to a timing sequencegenerator 4-56 and the other of which leads to the integrator 4-58through the veto AND gate 4-60 and the line 4-62. As will be explained,there will be no output from the gate 4-60 unless there is an input at4-52 and no input at 4-64. The pulse 4-54 passes to the integrator 4-58(assuming that it is not blocked by the veto AND gate 4-60) andthereafter is converted into a ramp-and-pedestal shaped pulse 4-66 whichappears at the line 4-68 from the time 2 to the time t, as best shown ingraph E of FIG. 4B. This is in accordance with the explanationpreviously given in connection with the apparatus 3-10, the ramp portionof the signal being discontinued at the time t, which is when the signal4-54 is cut off. The condenser means in the integrator 4-58 retain theircharge to establish the pedestal or plateau of the signal for the timewhich is chosen by other considerations of the circuit. The amplitude ofthe pedestal of the pulse 4-66 represents the half-height duration ofthe particle pulse 4-30. The greater the duration of the pulse 4-30, orany pulse which is processed by the apparatus, the higher will be thepedestal of the signal from the integrator 4-58.

The signal 4-66 is applied to the broken line block 4- 70, this blockcomprising elements forming a minimum duration memory circuit identicalto that of circuit 3- 10. The output of this circuit 4-70 appears at4-72 and such output comprises a DC voltage proportional to the minimumduration. This is the lower of the two broken line wave forms of graph Edesignated 4-74. The details of operation of this circuit have beenpreviously explained. From such explanation, it is understood that itcontrols whether or not the pulse output of the integrator 4-58 (waveform 4-66) is effective to permit the particle pulse 4-30 to passthrough the last analog gate 4-76 to the output terminal 4-78. If theamplitude of the pulse 4-66 exceeds the voltage level of the line 4-72by an amount controlled by the gain of the adjustable gain amplifier4-80, the upper level being shown by the upper one of the broken linewave forms of graph E of FIG. 4B, namely 4-82, then the analog gate 4-76will be closed, that is, the precision electronic switch which comprisesthis gate will be opened. Under these circumstances, no signals can getthrough to the output terminal 4-78.

The minimum duration memory circuit 4-70 is composed of an inputcomparator 4-84 operating into one terminal 4-86 of an AND gate 4-88whose output 4-90 is applied to the shorting means of the integrator4-92. The constant current supply 4-94 may consist of a constant voltageand fixed resistor, the output of which is applied to the integrator4-92 to cause the voltage at the output 4-72 of the integrator 4-92 torise very slowly. in graph E the slope is exaggerated in the wave form4-74.

Each time that the comparator 4-46 produces a pulse 4-54 at the line4-52, the timing sequence generator 4- 56 generates a train of timingpulses, these being the pulses 4-96 (graph G), 4-98 (graph H) and 4-100(graph l). The leading edge detector 4-102 receives signal on line 4-103and produces a spike 4-104 (graph F) at the time which is coincidentwith the leading edge of the pulse 4-54, and assuming that this spikepasses through the veto AND gate 4-106 it will trigger the one-shot (A)multivibrator 4-108.

It will be recalled that the equivalent element 3-102 of the sensor 3-10was a trailing edge detector in order to minimize the timing cycle, itonly being necessary that the ramp pulse reach its final value beforetriggering the strobing one-shot. The leading edge of the pulse 4-96 isused in the case of the apparatus of FIG. 4A to cause the strobing pulseto occur more simultaneously with the doubly delayed particle pulse4-154.

The output of one-shot 4-108 appears at 4-110 and becomes input A to thelogic control circuit 4-16. It also is applied to the trailing edgedetector 4-1 12 which produces a spike at the time t, which is similarto, but considerably delayed from the spike 4-104. This spike occurs atthe line 4-116 and is the input to the second one-shot (B)multivibrator4-ll8. The output of the multivibrator 4-118 is therectangular wave 4-98 (graph H) between the times t, and t-, and itappears at the line 4-120. This line has a plurality of branches whichshould be examined for the moment.

One connection from the line 4-120 extends to the veto input 4-122 ofthe veto AND gate 4-106; another branch 4426 becomes the B input to thecontrol logic circuit 4-16; the branch 4-64 extends to the veto input ofthe veto AND gate 4-60 as previously mentioned; one branch extends tothe input 4-127 of the next trailing edge detector 4-131; one branch4-132 extends to an input of the AND gate 4-130; and finally, the branch4-132 extends to the second input of the AND gate 4- 88 in the minimumduration memory circuit. Completing the complement of the timingsequence generator 4-56, the trailing edge detector 4-131 producesanother spike similar to 4-104 at the time t which appears at the input4-136 of the one-shot (c) multivibrator 4- 138. The output of thismultivibrator is a short rectangular pulse 4400 (graph I of FIG. 48)that extends from the time t, to the time i and is applied at 4-140 tothe shorting means of the integrator 4-60 and via path 4-124 to thepulse stretcher 4-26 for reset purposes, and also becomes the C input at4-142 to the control logic circuit 4-16.

The output pulse 4-98 from the one-shot multivibrator 4-118 is delayedby the duration i to 2, due to the action of the trailing edge detector4-112 which provides a trigger pulse only at the end of the output pulse4-96, that is, at the time t The pulse 4-98 appears at 4- 132, the inputto the AND gate 4-88, after the output from the integrator 4-58 ceasesto increase. This occurs while the pulse 4-66 in graph E is operating onthe pedestal or plateau portion. If, at this time, the voltage at 4-68is less than the voltage at 4-144, this latter being one of the inputsto the comparator 4-84, then the comparator 4-84 has an output at 4-86which combines with the signal at 4-132 to produce an output at 4-90.This output is shown as a small pulse 4-146 between the times 1 and tthe beginning of the pulse 4-98. This is shown in graph K of FIG. 4B.When this happens, the shorting means of the integrator 4-92 isenergized for the period of time that the pulse 4-146 occurs, causingthe voltage at the output of the integrator 4-92 to decrease. This isshown in the graph E of FIG. 48 at the time t where the broken line 4-74drops slightly. When the voltage of the integrator 4-92 falls below thevoltage 4-66, at the time I the output from the comparator 4-84disappears, removing the energizing signal from the AND gate 4-88 andthe shorting means input 4-90 causing the integrator 4-92 to hold at thenew value which is that of the pulse 4-66.

If the pulse 4-66 is larger than the voltage at 4-l44, however, thecomparator 4-84 does not have an output, the shorting means of theintegrator 4-92 is not activated, and the voltage 4-74 is not efiected.There would be no output signal 4-146. in this manner, since only pulseshaving durations shorter than the previous minimum causes the integratoroutput at 4-72 to decrease, the voltage 4-74 at 4-72 is proportional toand represents the duration of the shortest pulse previously processed.

The voltage 4-74 appearing at the line 4-72 is applied to the adjustablegain amplifier 4-80 which has a gain slightly greater than unity. Thispermits pulses which are slightly longer than the previous minimum to beaccepted by the system and routed to the output tenninal 4-78. This isaccomplished by comparing the time-related pulses which appear at thepath 4-68 with the DC voltage on the path 4-148. The voltage on the path4- 148 is the broken line wave form 4-82 of graph E of FIG. 4B and thevoltage at 4-68 is, of course, the output pulse 4-66 of the integrator4-58. If a pulse is slightly longer in time than the percentageadjustment set by the amplifier 4-80 it will remove the output from thecomparator 4-150. This provides a slight increase over the level 4-74 totake care of slight variations in the time duration of pulses. Onlypulses having a duration shorter than that represented by the voltage4-82 will be accepted.

Assuming that the analog gate 4-20 is on, acceptably short particlepulses from the input terminal 4-12 will always pass through the analogsignal delay means 4-24 and 4-152 and appear without wave form change asthe pulse 4-154 at the line 4-156 (graph L, FIG. 4B). Whether or not theanalog gate 4-76 permits these pulses to be impressed upon the outputterminal 4-78 is determined by the state of the logic signal appearingat line 4-158, this being the output signal 4-160 (graph M, P10. 48)from the AND gate 4-130. This, in turn, is dependent upon whether or notthe pulse duration does or does not exceed the value which is apredetermined percentage greater than the duration of the previousminimum duration pulse, the latter being established by the minimumduration memory circuit 4-70, and the percentage leeway beingestablished by the amplifier 4- 80 and its gain adjust means 4-162. Italso depends upon the state of the logic or timing signal appearing onthe line 4-128. If there is a signal at 4-164 out of the comparator4-150, indicating that the duration of the pulse being analyzed is lessthan the maximum established by the amplifier signal 4-82, and theoneshot 4-118 has an output, the analog gate 4-76 will conduct and theparticle pulse 4-166 (graph N, H6. 43) will appear at the outputterminal 4-78.

The purpose of the one-shot multivibrators 4-108, 4- 118 and 4-138 is toprovide a timing sequence initiated at the instant that the delayedparticle pulse on the line 4-50 exceeds the fractional height determinedby the attenuator 4-40. This initial time in the graphs is at Theone-shot multivibrator 4-108 is adjusted to contribute somewhat lessthan the total delay of the analog delay means 4-152 if it is desired tohave the complete particle pulse appear at the output terminal 4-78. Thepulse duration from the one-shot multivibrator 4-118 is accordinglysomewhat longer in time than the longest legitimate particle pulseexpected. The trailing edge of the pulse 4-96 is detected by thedetector 4-112 which in turn triggers the one-shot multivibrator 4-118to produce the rectangular wave pulse 4-98 from t; to The duration ofthis pulse 4-98 determines how long the analog gate 4-76 conducts and istherefore adjusted to allow passage of the pertinent portion of thesignal through that gate, providing that there is a signal on the line4-164 as explained above.

The output signal 4-98 from the one-shot multivibrator 4-118 alsoappears on the return path 4-122 to the veto input of the gate 4-106,and also on the veto input of the gate 4-60 by way of the line 4-64. ltis also routed to the AND gate 4-130 as explained above. Since by thetime that the one-shot multivibrator 4-118 is triggered at the time ithe pulse 4-54 from the comparator 4-46 has subsided, the pulsestretcher 4-26 may be reset by the output of the one-shot multivibrator4-118 by way of the line dashed 4-124. Alternately, the pulse stretcher4-26 may be reset by an output via path 4-125 from the one-shotmultivibrator 4-138. This alternate method is illustrated by solid line4-125 of FIG. 4A and the waveforms of FIG. 4B.

The output pulse from the one-shot multivibrator 4- 118 is applied bythe line 4-64 to the veto AND gate 4- 60 to prevent false operation,which could happen if a second pulse follows closely after the one beinganalyzed, causing the comparator 4-46 to have a second output before theanalysis of the first is completed. This also occurs in connection withthe veto AND gate 4-106 which is disabled and prevents further signalsfrom energizing the timing sequence generator for the period of timethat there is an output from the one-shot multivibrator 4-118, that is,from the time t; to 1-,.

The presence of the analog gate 4-20 and its control logic circuit 446is to prevent the sensor circuit 4-10 from being turned on and madereceptive to a new signal from the input terminal 4-12, unless theinstantaneous voltage at the input terminal has dropped below a certainthreshold. This ensures that the next pulse analyzed will be a completeone and not a partial pulse. If a pulse which follows one that has justbeen analyzed is quite close, then the sensor circuit 4-10 may be turnedon while a partial pulse is at the input terminal. This pulse is seen asa truncated pulse possibly of very short duration. When this occurs thevoltage at 4- 72 will be dropped down unnecessarily and the apparatuswill be inoperative until the constant current source 4-94 can chargethe integrator 4-92 back up to a level representing the duration of thefastest acceptable pulses. This requires useless time.

The operation of the control logic circuit for preventing this isdescribed in connection with FIG. 4C. The block designated 4-16 in FIG.4A is comprised of a circuit including the comparators 4-170 and 4-172;the OR logic elements 4- 174, 4-176 and NOR logic element 4-178; the ANDgate 4-180; the reset-set flip-flop 4-182', and the electronic switch4-184 which preferably is a field effect transistor. Each of thecomparators has a reference voltage source connected through a resistorto its positive input terminal as shown at 4-186, 4-188, 4-190 and4-192. The voltage level at the respective inputs is adjusted by varyingthe effective resistance of the resistors 4-190 and 4-192. The fourinput lines from externally of the block 4-16 are as identified in H6.4A and are so designated. The output line is also the equivalent of thesame element in FIG. 4A. These lines are the equivalent of thosesimilarly numbered but with prefix 3" in FIG. 3A.

The condition of no signals at the lines 4-110, 4-126 and 4-142 and atthe input line 4-14 produces a logic l signal at the input line 4- 194leading to the C (CLEAR) input of the RS. flip-flop 4-182. The output Owhich is line 4-196 leading to the AND gate 4-180 is then in a logic 0state, so that there is no output from the AND gate 4-180. This is alogic 0. The OR element 4-176 has its inputs 4-198 and 4-200 at logic 0,that is, without signals, and hence its output line 4-202 will be alsowithout signal. Since this is the negative input to the comparator 4-172there will be a positive output at the line 4-18. This is applied to thebase of the f.e.t. 4-184 of analog gate 4-20 so that the transistorconducts. Thus, the analog gate 4-20 is also turned on, and the sensorapparatus 4-10 is in condition to pass incoming pulses to the pulsestretcher 4-26 and the analog signal delay means 4-24 for analysis.

During the period of time that a signal is being processed in theapparatus 4-10, there will be a signal (logic 1) present at one of theinput lines to the OR element 4-174. This will be obvious from thedescription of FIGS. 4A and 4B and the period of time involved is t to8- Thus, the output of the QRelernentf-iflj will comprise a signal(logic 1) which in turn produces a signal (logic 1) at the output 4-202of the OR element 4-176 causing the comparator 4-172 to have a negativeoutput at 4-204. This turns off the f.e.t. 4-184 and closes the analoggate 4-20 preventing the transfer of any further signals arriving at theinput terminal 4-12 to the sensor circuit 4-10. A signal (logic 1) atthe output of the OR element 4-174 also produces a signal (logic 1 atthe Q output 4-196 of the flip-flop 4-182.

If, at the end of the processing time when the output of the R element4-174 reverts to a no signal condition (logic 0), the input terminal4-14 is above the positive voltage set by the reference voltage source4-188 and the resistor 4-192, and being connected to the negative inputterminal of the comparator 4-1'70, the comparator will have a positiveoutput at the line 4- 206. This signal applied to the OR element 4-178produces a logic 0 output from the OR element 4-178 at the line 4-194 asa result of which the 0 output 4-196 of the flip-flop 4-182 will remainat a condition of signal output (logic 1). The presence of this signaland the positive output from the comparator 4-170 will produce a signal(logic 1) at the output line 4498 of the AND gate 4-180. The output ofthe OR element 4- 176 will remain at logic 1 and the f.e.t. 4-184 willremain off until the incoming signal at 4-14 falls below the thresholdat which time the output of the AND gate 4-180 reverts to no signal(logic 0) and the f.e.t. 4-184 will again be turned on, and signals fromthe terminal 4- 12 can be accepted by the apparatus 4-10.

Also, when the output of the comparator 4-170 reverts to logic 0, the ORelement 4-178 will have a signal output (logic 1) and the 0 output 4-196of the flip-flop 4-182 will revert to a logic 0. Thus, all componentsare returned to their initial or cleared state, and a new signal will beaccepted by the apparatus for processing.

From the above description of the control logic circuit 4-16, it will beseen that the output of the comparator 4-170 controls whether or notthere will be a signal accepted. During the period of time that thesignal 4-30 appears at the terminal 4-12, that is between the times t,and 1 this comparator 4-170 will produce a signal 4- 208 which willpermit the acceptance of any signals to circuit 4-10. At all other timesthat there is an input at the lines 4-110, 4-126 or 4-142 no signalswill be accepted. As soon as the lines clear, the sensor 4-10 willaccept signals, providing the threshold of 4-188 is not exceeded by anysignal at the input 4-14. The presence of this signal keeps the analoggate 4-20 closed.

Consider now the pulse 4-34 which, as seen, has a time duration of t, toi substantially greater than the duration of the pulse 4-30. Theprocessing of this pulse proceeds exactly as described in connectionwith the pulse 4-30, so that the same steps occur in the process. GraphB shows the stretched pulse 4-212 and, by broken lines, the pulses 4-30'and 4-34 which appear at output 4-22 of analog gate 4-20. Pulse 4-34 istruncated by the opening of gate 4-20 at time Graph C shows theattenuated pulse 4-214 and the delayed pulse 4-216 superimposed. Theoutput of the comparator 4- 46 is shown at 4-218 in graph D, and theintegrated ramp and pedestal wave form 4-220 is shown in graph E. Sincethe duration of the original pulse 4-34 was greater than the pulse 4-30,the amplitude of the pedestal portion of the pulse 4-220 is greater thanthat of the pulse 4-66, and in this case is shown to exceed the upperlimit established by the voltage 4-82 at the time I The tinting sequencegenerator 4-56 operates without change because it starts at the time rwhich is the leading edge of the comparator output 4-218. Thus, thepulses 4-222, 4-224, 4-228, and 4-232 are generated as shown. It will beseen that there is no pulse equivalent to the pulse 4-146 which wouldhave occurred at the time I because the pulse 4-220 is larger than thevoltage which appears at the line 4-144 so that the comparator 4-84 hasno output. The gate 4-76 remains closed, and no pulse equivalent to thepulse 4- appears, so that the pulse 4-34 is rejected.

In FIG. 5 there is illustrated apparatus designed to improve theoperation of the sensor 3-10 by replacing the manual gain adjustmentmeans 3-162 by an automatic gain adjustment means. In FIG. 5, so much ofthe block diagram of FIG. 3A is reproduced as necessary to show thechanges brought about by the substitution. The wave forms of FIG. 38will not be materially changed except that the two broken line waveforms 3- 74 and 3-82 of graph E may not be quite the same distanceapart. In FIG. 3B, the distance between these two lines is constant,representing a pre-set gain which is applied to the amplifier 3-80 inorder to establish the tolerance of duration above the maximum durationwithin which pulses will be accepted. The use of an automatic gainadjustment means will cause this distance to vary automatically inaccordance with the conditions established by certain settings of thecircuit.

If the manual adjustment 3-162 of sensor 3-10 is set at a given setting,some percentage of pulses will be rejected on the basis of pulseduration. The gain setting is thus an independent variable, and thepercentage of pulses passed is a dependent variable. In the case of theautomatic gain adjustment circuit of FIG. 5, there is a sensitivityadjust means 3-307 which is set at a given setting, that is, apercentage of pulses to be passed. The percentage passed will then bethe independent variable, and the tolerance will become the dependentvariable, adjusting itself to meet the conditions set by the sensitivityadjust means.

The adjustment of a control of the sensitivity of the circuit in termsof percentage to be accepted or rejected is a more readily graspedconcept and hence easier to explain to operators. It is thus more likelyto be set at an optimum condition.

The apparatus of FIG. 5 works on a feed-back principle, comparing somefraction of the total number of pulses processed per unit time with thenumber of pulses actually passed or rejected per unit time, and usingthe error signal so developed to cause the pulse duration tolerance towiden or narrow in order to minimize this error.

A pulse which occurs with each particle pulse is applied by way of theline 3-128 and the line 3-310 to the positive pump 3-300. This pulsecomprises the strobing pulse, but the pulses could be obtained fromother parts of the sensor 3-10. The primary consideration is that thereis one pulse output for each particle pulse being processed by thedevice. The negative pump 3-301 is driven by the pulses derived from theoutput of the AND gate 3-130 on the line 3-158. These pulses are onlythe ones which are permitted to pass. The positive and negative pumpsare rate meters. Their respective outputs are applied by means of thelines 3-302 and 3- 303 to the summing means 3-304 from whence theresulting signal is applied through the low pass amplifier 3-306 by wayof the lines 3-305 and 3-311 to the automatic gain adjust circuit3-162'.

Through the sensitivity adjust means 3-307 and the control paths 3-308and 3-309, the pumps are adjusted so that the summation of their outputsis equal to zero or substantially zero. (The phrases positive" andnegative" pumps merely signifies that these circuits have an oppositesense.) They could easily be adjusted by adjusting the chargingcapacitors in the respective circuits.

Thus, if the percentage of pulses which one wishes to pass is 50percent, then there would be twice as many pulses on the line 3-310 asthere are on the line 3-158, and for a zero error voltage, the pumpswould be adjusted so that each pulse applied to the negative pump 3-301causes twice the charge on the line 3-303 as each pulse applied to thepositive pump 3-300. The summing means, which may be a simple inputterminal to an integrator, the integrator being the low pass amplifier3- 306, will then see a zero error signal and the automatic gain adjustmeans 3-162' will not change the tolerance between the levels 3-74 and3-82. The sensitivity adjust means 3-307 could be a type of switchingdevice that makes the pump capacitor in the pump 3-301 twice the size ofthe pump capacitor in the pump 3-300.

If it is assumed for purposes of example, that the output of the lowpass amplifier 3-306 is such that the gain of the automatic gain adjustmeans 3-162' is too high, then the level of the broken line 15-82 ofgraph E of FIG. 3B is much higher than the level of the broken line 3-74. Assume further that not only is this too high, but is twice as highas the level 3-74. Thus, pulses which have durations twice the lowerlevel will be accepted and produce output pulses at 3-78. In otherwords, the tolerance is now 100 percent. Practically, about 90 percentof the pulses may be accepted, so that there are ninety percent as manypulses at 3-158 as there are at 3-310. Additionally, the negative pump3-30] is producing twice as much charge per pulse as the pump 3-300.Accordingly, the output at 3-303 will be much greater than that at 3-302and they will not balance at the summing means 3-304. Instead, the lowpass amplifier 3-306 will have a large output, and it will be coupled insuch phase with the automatic gain adjust means 3-162 that this willproduce a voltage at 3-311 which will reduce the gain of the automaticgain adjust means 3-162'. The upper voltage level 3-82 will thereforedescend closer to the level 3-74 and this will continue until there isonly a slight unbalance left at 3-305s The system will tend to stabilizeat a condition where approximately 50 percent of the pulses get through.This value is approximate because of the practical considerations of thelow pass amplifier 3-306. The criterion is its time constant whichshould be chosen to make the fluctuation as small as desired withoutinordinately increasing the time within which equilibrium is reached.

The low pass amplifier 3-306 could be an integrator, as stated, whichwill thus have a substantial gain at do The automatic gain adjust means3-162' could be an electronic analog divider of a type such as describedin The Microelectronics Data Book, 2nd Edition, published by theMotorola Co. using their analog multiplier Motorola MC 1595L. The path3-312 would be connected to its "numerator" input, and the path 3-3 1 1would be connected to its "denominator" input. Hence, the larger thesignal from the low pass amplifier 3-306, the smaller would be the gainthrough it. The error signal would always be great enough to keep thegain at least as high as unity. Obviously, if the gain ever droppedbelow unity, all of the ramp and plateau pulses like 3-66 and 3-220would exceed the voltage on the path 3-148 and no pulses would getthrough. ln this case, only the positive pump 3-300 would have an outputand the gain of the automatic gain adjust means 3- 162' would riserapidly until it reached equilibrium from the other direction.

it will be obvious that variations can be made in all of the structuresdescribed herein without departing from the spirit or scope of theinvention as claimed hereinafter. As an example, the tolerance which isrepresented in FIGS. 38 and 48 by the difference between the voltagelevels 3-74 and 3-82 and the levels 4-74 and 4-82 need not beestablished by increasing the lower level through the use of theadjustable amplifiers 3-80 or 4-80. Instead, the latter could beomitted, and the electrical time signal pulses from the integrators 3-58 and 4-58 respectively could be slightly attenuated by an attenuatorinterposed in the line between the integrator and the comparator 3-150or 4-150. Means for adjustment could easily be furnished.

What it is desired to secure by Letters Patent of the United States is:

lclaim:

1. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising:

A. input terminal means and output terminal means having a channel forpassage of electrical signals between the terminal means with switchmeans in said channel to control the signals which appear at the outputterminal means, the input terminal means adapted to have said desirableand other particle pulses applied thereto,

B. means for measuring the duration of a particle pulse applied to saidinput terminal means at some predetermined fraction of the amplitudethereof and deriving a duration-measuring pulse of constant amplitudeand having the measured duration,

C. means for converting said duration-measuring pulse into an electricaltime signal pulse whose amplitude is proportional to the duration ofsaid duranon-measuring pulse,

D. means for producing a voltage level representative of the amplitudeof a prior electrical time signal pulse produced by a desirable particlepulse,

E. means for comparing the amplitude of said electrical time signalpulse with said voltage level and providing a first type of energizingsignal if the amplitude exceeds the said voltage level and a second typeof energizing signal if the amplitude of said electrical time signaldoes not exceed said voltage level,

F. means for modifying one of said voltage level and said last-mentionedelectrical time signal pulse prior to comparing the same to establish apredetermined tolerance enabling the production of the second type ofenergizing signals even if the amplitude of said electrical time pulseexceeds said voltage level to some extent, and

G. means for applying one of said types of energizing signals to saidswitch means in said channel to permit passage to said output terminalmeans of only electrical signals derived from desirable particle pulses.

2. The sensor as claimed in claim 1 in which said measuring meansgenerates said last-mentioned derived electrical signals, which compriseoutput pulses of predetermined duration but having the respectiveamplitudes of desirable particle pulses.

3. The sensor as claimed in claim 1 which includes means causing saidlast-mentioned derived electrical signals to comprise desirable pulsespassing through said channel.

4. The sensor as claimed in claim 1 in which there is a timing sequencegenerator for producing a strobing pulse in timed relation to saidelectrical time signal pulse, means connecting said generator with saidapplyin g means whereby said strobing pulse will operate said switchmeans in the channel to pass electrical signals when said second type ofenergizing signal is applied to said switch means but will not operatesaid switch means when said first type of energizing signal is appliedthereto.

5. The sensor as claimed in claim 1 in which the modifying meanscomprise means for adjusting the first-mentioned voltage level to asecond voltage level higher than said first-mentioned voltage level byan amount to establish said predetermined tolerance, and in which saidamplitude of said electrical time signal pulse is compared with the saidsecond voltage level.

6. The sensor as claimed in claim 5 in which said means for adjustingsaid first-mentioned voltage level for tolerance includes an amplifierand manual adjust means therefor.

7. The sensor as claimed in claim 5 in which said means for adjustingsaid first-mentioned voltage level for tolerance includes an amplifierand automatic adjust means therefor responsive to the relation betweenall particle pulses received by said sensor and the percentage particlepulses producing derived electrical signals passing to said outputterminal means.

8. The sensor as claimed in claim 7 in which means are provided topre-set the percentage.

9. The sensor as claimed in claim 5 in which means are provided fordropping said first-mentioned and second voltage levels to the amplitudeof said electrical time signal pulse after comparing same, but saiddropping means being operative only if the amplitude of said electricaltime signal pulse does not exceed the second voltage level, and saidfirst voltage level producing means including a memory for substantiallyretaining the first voltage level after dropping; until anotherelectrical time signal pulse arrives.

10. The sensor as claimed in claim 9 in which said first-mentionedvoltage level establishing means is an integrator and said droppingmeans include a comparator and a timing gate connected with the resetmeans of said integrator.

11. The sensor as claimed in claim 5 in which said first-mentionedvoltage level producing means comprise an integrator.

12. The sensor as claimed in claim I in which there is an analog gate insaid channel between said input ter minal means and the remainder ofsaid sensor and having a control circuit for opening and closing thesame, said control circuit having means for producing control signals toclose said gate in response to the existence of certain signalconditions in parts of said sensor.

13. The sensor as claimed in claim 12 in which said last-mentioned meansrespond to the presence of a particle pulse being operated upon by saidsensor to disable the channel to receive other particle pulses at saidinput until the operation is complete.

14. The sensor as claimed in claim 13 in which said sensor has timingsignal producing means driven by said particle pulse while beingoperated upon for producing said control signals.

15. The sensor as claimed in claim 13 in which said last-mentioned meanscomprise a timing sequence generator energized by said durationmeasuring means for producing said timing signals until afterapplication of said energizing signals to the switch means in saidchannel whereby to disable said sensor from receiving particle pulses atits input means until after application of said energizing signals.

16. The sensor as claimed in claim 12 in which said last-mentioned meansrespond to the presence of a voltage at said input means which exceeds apredetermined threshold to close said gate until said voltage at saidinput means subsides below said threshold, whereby only completeparticle pulses can pass through said analog gate.

17. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising:

A. input terminal means and output terminal means having a channel forpassage of electrical signals between the terminal means with switchmeans in said channel to control the signals which appear at the outputtemiinal means, the input terminal means adapted to have said desirableand other particle pulses applied thereto,

1. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising: A. input terminal means and output terminal means having achannel for passage of electrical signals between the terminal meanswith switch means in said channel to control the signals which appear atthe output terminal means, the input terminal means adapted to have saiddesirable and other particle pulses applied thereto, B. means formeasuring the duration of a particle pulse applied to said inputterminal means at some predetermined fraction of the amplitude thereofand deriving a duration-measuring pulse of constant amplitude and havingthe measured duration, C. means for converting said duration-measuringpulse into an electrical time signal pulse whose amplitude isproportional to the duration of said duration-measuring pulse, D. meansfor producing a voltage level representative of the amplitude of a priorelectrical time signal pulse produced by a desirable particle pulse, E.means for comparing the amplitude of said electrical time signal pulsewith said voltage level and providing a first type of energizing signalif the amplitude exceeds the said voltage level and a second type ofenergizing signal if the amplitude of said electrical time signal doesnot exceed said voltage level, F. means for modifying one of saidvoltage level and said lastmentioned electrical time signal pulse priorto comparing the same to establish a predetermined tolerance enablingthe production of the second type of energizing signals even if theamplitude of said electrical time pulse exceeds said voltage level tosome extent, and G. means for applying one of said types of energizingsignals to said switch means in said channel to permit passage to saidoutput terminal means of only electrical signals derived from desirableparticle pulSes.
 2. The sensor as claimed in claim 1 in which saidmeasuring means generates said last-mentioned derived electricalsignals, which comprise output pulses of predetermined duration buthaving the respective amplitudes of desirable particle pulses.
 3. Thesensor as claimed in claim 1 which includes means causing saidlast-mentioned derived electrical signals to comprise desirable pulsespassing through said channel.
 4. The sensor as claimed in claim 1 inwhich there is a timing sequence generator for producing a strobingpulse in timed relation to said electrical time signal pulse, meansconnecting said generator with said applying means whereby said strobingpulse will operate said switch means in the channel to pass electricalsignals when said second type of energizing signal is applied to saidswitch means but will not operate said switch means when said first typeof energizing signal is applied thereto.
 5. The sensor as claimed inclaim 1 in which the modifying means comprise means for adjusting thefirst-mentioned voltage level to a second voltage level higher than saidfirst-mentioned voltage level by an amount to establish saidpredetermined tolerance, and in which said amplitude of said electricaltime signal pulse is compared with the said second voltage level.
 6. Thesensor as claimed in claim 5 in which said means for adjusting saidfirst-mentioned voltage level for tolerance includes an amplifier andmanual adjust means therefor.
 7. The sensor as claimed in claim 5 inwhich said means for adjusting said first-mentioned voltage level fortolerance includes an amplifier and automatic adjust means thereforresponsive to the relation between all particle pulses received by saidsensor and the percentage particle pulses producing derived electricalsignals passing to said output terminal means.
 8. The sensor as claimedin claim 7 in which means are provided to pre-set the percentage.
 9. Thesensor as claimed in claim 5 in which means are provided for droppingsaid first-mentioned and second voltage levels to the amplitude of saidelectrical time signal pulse after comparing same, but said droppingmeans being operative only if the amplitude of said electrical timesignal pulse does not exceed the second voltage level, and said firstvoltage level producing means including a memory for substantiallyretaining the first voltage level after dropping; until anotherelectrical time signal pulse arrives.
 10. The sensor as claimed in claim9 in which said first-mentioned voltage level establishing means is anintegrator and said dropping means include a comparator and a timinggate connected with the reset means of said integrator.
 11. The sensoras claimed in claim 5 in which said first-mentioned voltage levelproducing means comprise an integrator.
 12. The sensor as claimed inclaim 1 in which there is an analog gate in said channel between saidinput terminal means and the remainder of said sensor and having acontrol circuit for opening and closing the same, said control circuithaving means for producing control signals to close said gate inresponse to the existence of certain signal conditions in parts of saidsensor.
 13. The sensor as claimed in claim 12 in which saidlast-mentioned means respond to the presence of a particle pulse beingoperated upon by said sensor to disable the channel to receive otherparticle pulses at said input until the operation is complete.
 14. Thesensor as claimed in claim 13 in which said sensor has timing signalproducing means driven by said particle pulse while being operated uponfor producing said control signals.
 15. The sensor as claimed in claim13 in which said last-mentioned means comprise a timing sequencegenerator energized by said duration measuring means for producing saidtiming signals until after application of said energizing signals to theswitch means in said channel whereby to disable said sensor fromreceiving particle pulses at its input means until after appliCation ofsaid energizing signals.
 16. The sensor as claimed in claim 12 in whichsaid last-mentioned means respond to the presence of a voltage at saidinput means which exceeds a predetermined threshold to close said gateuntil said voltage at said input means subsides below said threshold,whereby only complete particle pulses can pass through said analog gate.17. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising: A. input terminal means and output terminal means having achannel for passage of electrical signals between the terminal meanswith switch means in said channel to control the signals which appear atthe output terminal means, the input terminal means adapted to have saiddesirable and other particle pulses applied thereto, B. means formeasuring the duration of a particle pulse applied to said inputterminal means at some predetermined fraction of the amplitude thereofand deriving a duration-measuring pulse of constant amplitude and havingthe measured duration, C. means for converting said duration-measuringpulse into an electrical time signal pulse whose amplitude isproportional to the duration of said duration-measuring pulse, D. meansfor establishing a voltage level representative of the amplitudeequivalent to the maximum duration of desirable particle pulses, E.means comparing the amplitude of said electrical time signal with saidvoltage level and providing a first type of energizing signal if saidamplitude exceeds said level and a second type of energizing signal ifthe amplitude does not exceed said level, F. gating means providedbetween the comparing means and the switch means, G. strobing pulseproducing means connected with said switch means and coupled with saidduration measuring means to produce a strobing pulse at said gatingmeans in timed relation to a particle pulse applied to said inputterminal means, H. means for applying all particle pulses through saidchannel to said switch means in synchronized relation with said strobingpulse, I. means for applying said energizing signals to said gatingmeans, said gating means being constructed to pass said strobing pulseto operate said switch means to signal-passing condition only when saidsecond type of energizing signal is applied to said gating means,whereby said switch means will be activated to pass particle pulseswhose duration is less than said maximum duration.
 18. The sensor asclaimed in claim 17 in which there is a connection between said channeland said measuring means and analog signal delay means between saidinput terminal means and said connection whereby to delay application ofsaid particle pulses to said measuring means.
 19. The sensor as claimedin claim 17 in which said means for applying said particle pulsesthrough said channel comprise analog signal delay means in said channel.20. The sensor as claimed in claim 19 in which there is a connectionbetween said channel and said measuring means and at least a portion ofsaid analog signal delay means between said input terminal means andsaid connection whereby to delay application of said particle pulses tosaid measuring means.
 21. The sensor as claimed in claim 17 in whichsaid measuring means comprise circuitry to produce a sTretched pulse ofa duration substantially greater than the duration of the particle pulsefrom which the same is derived and having an amplitude over asubstantial portion of its duration which is said predetermined fractionof the amplitude of the particle pulse from which same is derived, andsecond means for comparing said last-mentioned particle pulse with saidstretched pulse during a period of time when the amplitude of saidstretched pulse is constant.
 22. The sensor as claimed in claim 21 inwhich analog signal delay means are provided to delay application ofsaid last-mentioned particle pulse to said second comparing means. 23.The sensor as claimed in claim 21 in which said means for applying saidparticle pulses through said channel comprise second analog signal delaymeans in said channel, said first analog signal delay means also beingin said channel and acting upon all pulses passing through the channel.24. An axial trajectory sensor for use with a Coulter type particleanalyzing apparatus in which particles passing through an apertureproduce desirable particle pulses whose amplitudes are most nearlyproportional to their respective sizes when passing closest to an axialtrajectory through said aperture and having thereby a certainapproximate duration, and in which particles passing through saidaperture on trajectories displaced from the axis will produce otherparticle pulses whose amplitudes are not necessarily proportional totheir respective sizes and whose durations tend to be greater than saidcertain approximate duration; said sensor acting to respond to saiddesirable particle pulses and not to respond to said other pulses andcomprising: A. input terminal means and output terminal means having achannel for passage of electrical signals between the terminals withswitch means in said channel to control the signals which appear at theoutput terminal means, the input terminal means adapted to have saiddesirable and other particle pulses applied thereto, B. means formeasuring the duration of at least a predetermined portion of a particlepulse applied to said input terminal means at some predeterminedfraction of the amplitude thereof and deriving a duration-measuringpulse of constant amplitude and having the measured duration, C. meansfor converting said duration-measuring pulse into an electrical quantitywhose value is proportional to the time duration of saidduration-measuring pulse, D. means establishing an electrical effect ofa standard equivalent to a maximum desired duration distinguishingbetween quantities produced by desirable and other pulses, E. meanscomparing said quantity with said standard and providing one type ofenergizing signal if the quantity exceeds the standard and a second typeof energizing signal if the said quantity does not exceed the standard,F. means for applying one of said energizing signals to the switch meansin said channel to permit passage to said output terminal means of onlyelectrical signals derived from desirable particle pulses, and G. ananalog gate in said channel between said input terminal means and theremainder of the sensor and having a control circuit for opening andclosing the same, said control circuit having means for producingcontrol signals to close said gate in response to the existence ofcertain signal conditions in parts of said sensor.
 25. The sensor asclaimed in claim 24 in which said last-mentioned means respond to thepresence of a particle pulse being operated upon by said sensor todisable the channel to receive other particle pulses at said input untilthe operation is completed.
 26. The sensor as claimed in claim 25 inwhich said sensor has timing signal producing means driven by saidparticle pulse while being operated upon for producing said controlsignals.
 27. The sensor as claimed in claim 25 in which saidlast-mentioned means comprise a timing sequence generator energized bysaid duration measuriNg means for producing said timing signals untilafter application of said energizing signals to the switch means in saidchannel whereby to disable said sensor from receiving particle pulses atits input means until after said application of said energizing signals.28. The sensor as claimed in claim 25 in which said last-mentioned meansalso respond to the presence of a voltage at said input means whichexceeds a predetermined threshold to close said gate until said voltageat said input means subsides below said threshold, whereby only completeparticle pulses can pass through said analog gate.
 29. The sensor asclaimed in claim 24 in which said last-mentioned means respond to thepresence of a voltage at said input means which exceeds a predeterminedthreshold to close said gate until said voltage at said input meanssubsides below said threshold, whereby only complete particle pulses canpass through said analog gate.
 30. The method of sensing between theparticle pulses caused by particles passing through a Coulter particleapparatus aperture on axis of the aperture and off the axis of theaperture, which comprises: A. measuring the duration of at least apredetermined portion of each particle pulse at a predetermined fractionof the amplitude thereof and deriving therefrom a duration-measuringpulse of constant amplitude and having a predetermined duration, B.converting the duration-measuring pulse into an electrical time signalpulse whose amplitude is proportional to the time duration of saidduration-measuring pulse, C. establishing a signal level equivalent to amaximum desired duration distinguishing between electrical time signalpulses produced by particle pulses which pass through the aperture offthe axis and other particle pulses of shorter duration, D. comparing thesaid electrical time signal pulse with said signal level and obtainingan energizing signal of one type if the signal level is exceeded and asecond type of energizing signal if the signal level is not exceeded, E.deriving an electrical signal from each pulse, and blocking or passingsaid derived signal on the basis of whether it has produced anenergizing signal of the first type or the second type, respectively,and F. rendering said measuring step ineffectual for a substantial timeafter execution thereof whereby to block sensing of pulses arrivingwhile one is being processed.
 31. The method of sensing between theparticle pulses caused by particles passing through a Coulter particleapparatus aperture on axis of the aperture and off the axis of theaperture, which comprises: A. measuring the duration of at least apredetermined portion of each particle pulse at a predetermined fractionof the amplitude thereof and deriving therefrom a duration-measuringpulse of constant amplitude and having a measured duration, B.converting the duration-measuring pulse into an electrical time signalpulse whose amplitude is proportional to the time duration of saidduration-measuring pulse, C. establishing a signal level equivalent to amaximum desired duration distinguishing between electrical time signalpulses produced by particle pulses which pass through the aperture offthe axis and other particle pulses of shorter duration, D. sensing theduration of the shortest duration electrical time signal pulse occurringprior to the particle pulse being sensed and adjusting the signal levelin response thereto, said shortest duration electrical time signal pulsebeing one which has not exceeded the signal level prevalent at the timeof its occurrence, E. comparing the said electrical time signal pulsewith said signal level and obtaining an energizing signal of one type ifthe signal level is exceeded and a second type of energizing signal ifthe signal level is not exceeded, and F. deriving an electrical signalfrom each pulse, and blocking or passing said derived signal on thebasis of whether it haS produced an energizing signal of the first typeor the second type, respectively.
 32. An axial trajectory sensor for usewith a particle study apparatus in which particles pass through adetecting zone having an axis for producing particle pulses, theparticles, when passing closest to an axial trajectory through thedetecting zone, producing desirable particle pulses having amplitudeswhich are most nearly proportional to the respective sizes of theparticles, and also thereby having a certain approximate duration, andin which particles passing through the detecting zone on trajectoriesdisplaced from its axis will produce other particle pulses havingamplitudes which are not necessarily proportional to their respectivesizes and having durations which tend to be longer than said certainapproximate duration; said sensor being constructed with an input andarranged to respond to said desirable particle pulses in a first mannerand to respond to said other particle pulses in a second manner andcomprising: A. means for measuring the duration of at least apredetermined portion of a particle pulse applied to the input of saidsensor at some predetermined fraction of the amplitude thereof andderiving a duration-measuring pulse having the measured duration; B.means for establishing an electrical standard value representative ofthe duration of a prior desirable particle pulse; C. means for comparingeach said duration-measuring pulse with said standard value and forproviding a first type of energizing signal if said standard value isnot exceeded and a second type of signal if said standard value isexceeded; D. means for modifying one of said standard value and saidduration-measuring pulse prior to their comparison to establish apredetermined tolerance enabling the production of the first type ofenergizing signal even if the duration-measuring pulse exceeds saidstandard value to some extent, and E. means coupled to receive saidfirst and second types of signals for generating, respectively, thefirst and second manners of response.